Opera (9.5) has an option to allow scripts to detect right-clicks, but it is disabled by default. Furthermore, Opera still doesn’t allow JavaScript to disable the browser’s default context menu which causes a usability conflict.

You can try to disable by disabling the mousedown event filtering only for Opera, but I do suggest a lot testing on this.

To complete just an extra note: this function does not bind the documents inside an iframe, so in this case you should complete the function by disabling the context menu in the iframe document as well.